Conventional methods for measuring cognitive load, include:                subjective measures, such as self-rating scales;        physiological techniques, such as pupil dilatation and heart rate;        task or performance based measures, such as critical error rates; and        behavioral measures, such as speech disfluencies, self-talk etc.        
The are a number of problems with these methods for measuring cognitive load, including:                some of the methods are intrusive and disrupt the normal flow of interaction;        some of the methods are physically uncomfortable for the user;        when conducted in real-time, some of the methods are labour-intensive;        the data quality is potentially unreliable outside laboratory conditions; and        the data quality can be affected by outside factors, such as user's stress level.        
At this time there is no way to objectively quantify cognitive load that can be applied uniformly across fields as a standard, or to allow comparison between subjects. In fact, historically, the most consistent results for cognitive load assessments have been achieved through self-rating subjective measures; these allow users to describe in fine detail, their perceived level of cognitive load induced by various types of task.
A multimodal interface is an interface where the user is able to select one or more unimodal input or output devices to operate the interface. Use of each device represents a single mode of input or output. Most interfaces offer at least two input modes, keyboard and mouse. Multimodal interfaces typically offer more options, including for instance voice recognition and other less common modalities.